Adjustable attenuator



March 1, 1960 A. BOWMAN ADJUSTABLE ATTENUATOR Filed June 2'7, 1957 N7//////////// \Q Q Q Q Q- w Q Q NW ///2 In I w NMQ Q 5mg m .ll 5% 6 Km Q.

B 6 a J y Attorney United States Patent ADJUSTABLE ATTENUATOR 7 Alfred Bowman, Canvey Island, England, assignor to Egen Electric Limited, (Charfleet Industrial Estate),

This invention relates to attenuators for use in coaxial transmission lines, for example for insertion between a television receiver and its aerial, and in particular to attenuators of the type comprising a pi-type resistance network, which will attenuate signals whilst substantially preserving the impedance characteristics of the line into which it is inserted.

An object of the present invention is to provide an attenuator of the above type which is easily manufactured and assembled and in which inherent shunt capacity coupling is negligible for frequencies at present in common use for television transmissions.

A further object of the invention is to provide such an attenuator which may be adjusted or pre-set to any one of a number of pro-selected degrees of attenuation.

A feature of the present invention is an attenuator for a co-axial transmission line, comprising an outer metallic casing constituting an outer conductor, each end of the casing encircling a terminal for the innner conductor, two resistance elements of substantially disc form, one near one end of the casing and the other near the other end thereof, which resistances are connected in shunt between the outer conductor and the respective inner conductor terminals, and a third resistance, also of substantially disc form, housed between the other resistance elements and connected in series between the inner conductor terminals. 7

One embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure l is a circuit diagram of a known pi-type attenuator network, Figure 2 is a sectional side elevation of one form of the invention and Figure 3 is an end elevation of a part shown also in Figure 2. Fig. 4 is a crosssectional view taken on the line 44 of Fig. 2.

In Figure 1, three resistances a, b, and 0, form a pi type attenuator network, the portion of resistance 0 connected in circuit being adjustable. It is important that the capacity d be as small as possible and this governs the practical design of the attenuator. The characteristics of the network are such that a slight mismatch occurs when the effective value of c is varied, but this is found to be tolerable over the changes in effective value of c required to efiect attenuation of, say, from six to thirty-six deci bels. (The terms input and output," indicated Figure 1, are, of course, interchangeable.)

The attenuator of Figure 2 has an outer conductor in the form of a cylindrical metallic casing comprising two readily assembled co-axial parts 1 and 2, the latter having an internal thread engaging an external screw thread on part 1. The casing is provided with co-axial end connectors, in which an end of the casing (outer conductor) encircles a terminal for the inner conductor of the transmission line. Part 1 of the casing houses resistance elements b and c which are in the form of a carbon track deposited on a disc of insulating material, similar to that often used in volumecontrol potentiometers for radio receivers. Part 2 of the casing houses resistance element a, also of disc form. The location of the carbon tracks a,

b and c of the resistance elements on the respective facesof discs 2 and 3 is best shown in Fig. 2, the carbon tracks being indicated by the criss-cross hatching. Elements b and c are mounted against opposite faces of a disc 3 of suitable insulating material by means of a plug-like inner conductor terminal 4, whose inner end is passed through central openings in elements b and c and disc 3 and is turned over, in rivet fashion, against a metallic washer 5. This fixing also effects the direct electrical connection between elements b and c as shown in Figure 1. In a somewhat similar manner, resistance element a is mounted against one face of another insulating disc 12 by means of a socket-like inner conductor terminal 10 and another metalic washer. The three resistance elements are thus grouped in two separate subassemblies, one near each end of the casing.

The carbon tracks of resistance elements a and b extend over substantially the whole of the face of the insulating material on which they are deposited. It will be seen, therefore, that resistance element a bears against a shoulder on inner conductor terminal 10 and also, around its periphery, against an internal flange in part 2 of the casing, and is thus electrically connected in shunt between inner conductor terminal 10 and the outer conductor, i.e. the metallic casing. Resistance element b is electrically connected in somewhat similar manner, in shunt between inner conductor terminal 4 and the metallic casing. The carbon track of resistance element 0 is in the form of a peripheral band having two ends separated by a small gap, one end being connected by an integral radial portionwith a central annular portion which bears against the metallic washer 5. The element c is thus electrically connected at one end to the inner 'conductor terminal 4, whereas its other end is free (as indicated in Figure 1), and an adjustable portion of element 0 is connected in circuit between inner conductor terminals 4 and 10, depending on the point of engagement of a tapping contact as will be explained. The track of element 0 is coated in such a way that the element is graded approximately logarithmically, so that by varying the effective value by equal mechanical (rotary) displacements of the tapping contact, the attenuation effected by the network may be varied in approximately equal steps. The shape of the carbon track of element 0 is preferably determined by stamping out the track, together with the disc on which it is deposited, to the required shape,and this latter mode of construction is indicated in Figures 2 and 4. g V

A cylindrical hole in part 1 of the casing houses a metallic locating pin 6 which is split longitudinally so that its resilience holds it firmly in position. Pin 6 passes through peripheral slots in elements b and c and disc 3;, and thereby prevents rotation of the elements within the casing,,but it is arranged that pin 6 neither forms an electrical connection between the elements nor between element c and the metallic casing.

Part Zof thecasing has a tubular end 8 provided withlongitudinalslots such as 9 and is encircled by a circlip 11. 'In conjunction with the socket-like shape of inner conductor terminal 10, end 8 is adapted to receive and grip a conventional plug-like co-axial connector (for example, one connected to a co-axial line from a television receiving aerial).- The outer conductor at the other end of the casing and the inner conductor terminal 4 are shapedto co-operate with a conventional socket-like coaxial connector (for example, on a television receiver) of a type adapted to mate with said plug-like connector, so that the attenuator may be inserted into a co-axial line by a simple plugging-in operation.

A cylindrical member 13 of electrically insulating material is housed within the casing and has a central longitudinal opening 15. This member is also shown in and elevation in Figure 3 (viewed from the left of the member in relation to its position in Figure 2). The member 13 is made hollow (or substantially so) in order to maintain at a low value (i.e. close to unity) the dielectric constant of the material located directly between the inner conductor terminals 4 and 10 and thereby assists in minimising the inherent shunt capacity indicated at d in Figure l. Member 13 is provided around its periphery with a plurality of longitudinal slots 14 extending a short distance from its left-hand end (in relation to Figure 2), and each of these slots is adapted to accommodate the locating pin 6 depending on the rotary position of the member, relative to the metallic casing, as Will be explained.

Cylindrical member 13 has a longitudinal slot 16, which is T-shaped in cross-section (see Figure 3) and extends over the length of the'member. In this slot is carried a longitudinal electrical conductor 17 (see Figure 2). It will be observed that in Figure 2 the rotary position of member 13, relative to the casing, is such that the locating pin 6 is shown as being accommodated in slot 16. This is merely to simplify representation of the parts in the drawing: in the use of the attenuator, the rotary position of member 13, relative to the casing, will be such that the pin 6 will be accommodated in one or other of the slots 14. The longitudinal conductor 17 is bent over at one end, 18, so as to form a tapping contact engaging the peripheral-band portion of resistance element 0. The other end of conductor 17 is in the form of a spider 19, which is shaped in the form of a cross when viewed in end elevation, the arms of the cross being located in transverse grooves in the right-hand end of member 13. Spider 1% is also of dished shape, as indicated in Figure 2, and bears resiliently against the riveted end of inner conductor terminal 10 to make electrical contact therewith in all rotary positions of the member 13. 'It will be seen that, in the embodiment illustrated, six slots 14 are provided in member 13, so that by inserting member 13 in the casing with locating pin 6 accommodated in any one of slots 14, six diiferent rotary positions of the member 13. relative to the casing, are provided, whereby a choice of 6 varying portions ofresistance element may be connected in series between inner conductor terminals 4 and 10, the circuit being (from left to right in Figure 2) terminal 4, adjusted portion of c, end 18 of conductor 17, through to end 19 of conductor 17, to terminal 10.

In the case of an attenuator specifically for connection between a television receiver and its aerial, such six positions are conveniently arranged so that attenuation may be effected in steps of approximately six decibels, from a minimum of about six decibels to a maximum of about thirty-six decibels, depending on how much of resistance element c is effectively included in the network.

. The member 13 is provided with indications around its periphery and these indications are arranged so that an indication appropriate to the relative position of member 13 may be viewed through anopening 7 in'part 1 of the casing. The indications serve to indicate the different degrees of attenuation effected by the network corresponding to whichever slot 14 is occupied by .pin 6. To as semble the attenuator, member 13 is inserted into part 1 of the casing so as to occupy the desired position relative thereto, and part 2 of the casing is then screwed on to part 1.

Projections are conveniently end of member 13 in order to maintain that'end substantially parallel with element 0. These projections are shown in Figure 3 as segmental portions angularly disposed by about 120 degrees clockwise and anti-clockwiseof slot 16, and they serve, in effect, to counterbalance the thickness of the end 18 of conductor 17.

provided on the left-hand- Various modifications may be made in the details shown without exceeding the scope of the invention. For example, the effective value of resistance element 0 may be made continuously variable instead of being adjustable in steps.

I claim:

1. An attenuator for a co-axial transmission line, comprising an outer metallic casing constituting an outer conductor, each end of the casing encircling a terminal for the inner conductor of the line, a pi-type network, three resistance elements of substantially disc form constituting said network and mounted within said casing transversely of its longitudinal axis, one of said elements being located near one end of the casing and another being located near the other end thereof, these two elements being connected in shunt between the outer conductor and the respective inner conductor terminals, the third resistance element being located near one of the other resistance elements, and comprising also a cylindrical member of electrically insulating material housed substantially coaxially withinthe casing, an electrical conductor carried by said cylindrical member and insulated from the casing, and means for locating said cylindrical member in any one of a plurality of different rotary positions, relative to the said casing, whereby the said electrical conductor may be caused to connect differently valued portions of said third resistance element in series between said inner conductor terminals.

2. An attenuator according to claim 1, in which said cylindrical insulating member is hollow throughout at least a substantial part of the region of its longitudinal axis.

3. An attenuator according to claim 1, comprising fastening means forming part of each said inner conductor terminal, two discs of electrically insulating material, one appropriate to each inner conductor terminal, said third resistance element and one of said other resistance elements being mounted against opposite faces of one of said insulating discs by the fastening means of one of said inner conductor terminals, and the remaining resistance element being mounted against one face of the other of said insulating discs by the fastening means of the other inner conductor terminal, whereby the resistance elements are grouped into two separate sub-assemblies which are retained against longitudinal movement within the assembled attenuator by means of said cylindrical insulating member and the said electrical conductor, in conjunction with said casing.

4. An attenuator according to claim 2, comprising fastening means forming part of each said inner conductor terminal, two discs of electrically insulating material, one appropriate to each inner conductor terminal, said third resistance element and one of said other resistance elements being mounted against opposite faces of one of said insulating discs by the fastening means of one of said inner conductor terminals, and the remaining resistance element being mounted against one face of the other of said insulating discs by the fastening means of the other inner conductor terminal, whereby the resistance elements are grouped into two separate sub-assemblies which are retained against longitudinal movement within the assembled attenuator by means of said cylindrical insulating member and the said electrical con-" ductor, in conjunction with said casing.

References Cited in the file of this patent UNITED STATES PATENTS 1,948,774 Siegel Feb. 27, 1934 2,510,614 Weber June 6, 1950 2,667,622 Weber Jan. 26, 1954 

